Various pitch shifting apparatuses which alter (or shift) a pitch of sound data, such as voice data and musical sound data, have been known. One of these pitch shifting apparatuses transforms given sound data from data represented in the time domain (time domain representation) into data represented in the frequency domain (frequency domain representation), identifies a frequency region which includes a peak spectrum of an amplitude spectrum based on the transformed sound data and shifts only amplitude spectra within the identified frequency region by a given amount evenly (for example, see U.S. Pat. No. 6,549,884 (FIGS. 3 and 4A to 4C)).
Generally, sound data includes two or more peak spectra with different frequencies and naturally amplitude spectra exist between two of the peak spectra (i.e., within intermediate frequency region between frequencies corresponding to the two peak spectra). However, according to the conventional apparatus mentioned above, the amplitude spectra in the intermediate frequency region are neglected and not reflected in the pitch-shifted amplitude spectra. As a consequence, the problem arises that the pitch-shifted sound may contain unnatural sound.